An exemplary process for manufacturing glass articles begins with the melting of raw feed materials, such as metal oxides, to form a molten glass. The melting process not only results in the formation of glass, but also the formation of various unwanted by-products, including various gases such as oxygen, carbon dioxide, carbon monoxide, sulfur dioxide, sulfur trioxide, argon, nitrogen, and water. Unless removed, these gases can continue throughout the manufacturing process, ending up as small, sometimes microscopic gaseous inclusions or blisters in the finished glass article.
For some glass articles, the presence of small gaseous inclusions is not detrimental. However, for other articles of manufacture, gaseous inclusions as small as 50 μm in diameter are unacceptable. One such article is the glass sheet used in the manufacture of display devices such as liquid crystal and organic light emitting diode displays. For such applications, the glass must have extraordinary clarity, pristine surfaces, and be essentially free of distortion and inclusions.
To remove gaseous inclusions from the molten glass, a fining agent or agents are typically added to the feed material. The fining agent can be a multivalent oxide, such as As2O3. As2O3 is converted into As2O5 during glass melting process. During fining stage, the following reaction occurs:As2O5→As2O3+O2(gas)The released oxygen forms gas bubbles in the molten glass, or melt. The gas bubbles allow other dissolved gases to be collected and rise to the surface of the melt, where it is removed from the process. The heating is typically performed in a high temperature fining vessel.
The fining temperatures for display-grade glasses can be as high as 1700° C. Temperatures this high require the use of specialized metals or alloys to prevent destruction of the vessel. Platinum or platinum alloys, such as platinum-rhodium are typically used. Platinum advantageously has a high melting temperature and does not easily dissolve in the glass. Nevertheless, at such high temperatures, the platinum or platinum alloy readily oxidizes. Therefore, steps must be taken to prevent contact between the hot platinum fining vessel and atmospheric oxygen. Other measures for reducing the oxidation of Pt during glass fining and delivery is highly desired as well.
Higher fining temperature of the molten glass is desired in order to obtain higher quality glass for a given residence time in the fining vessel. However, the material forming the fining system such as one made with a metal or a metal alloy, has a maximal operating temperature. Therefore, it would be desirable to have a glass fining system which provides a high fining temperature to the molten glass without exceeding the highest operating temperature of the metal.
The present invention satisfies this need.